The present invention relates to a method for the preparation of a vinyl chloride-based polymer, referred to as a PVC hereinafter, of high quality useful as a molding material of various kinds of shaped articles. More particularly, the invention relates to a method for the preparation of a PVC having a greatly improved particle configuration by preventing foams rising in the polymerization reactor during the polymerization reaction.
It is a trend in the industry of PVC manufacture in recent years that the process of polymerization is conducted by using a polymerization reactor having an increasingly larger and larger capacity and various means are undertaken for decreasing the time taken for completing a cycle of the batch-wise polymerization reaction in order to improve the overall productivity of the process. It is usual accordingly that the polymerization reactor is equipped with a reflux condenser with an object to quickly remove the heat of polymerization evolved as the polymerization reaction proceeds by circulating the vinyl chloride monomer therein.
A problem in this method of operating a reflux condenser for the removal of the heat of polymerization, however, is that foaming sometimes takes place on the polymerization mixture in the reactor so that the particles of the PVC resin produced may have a low bulk density with a foamy structure. The PVC resin product thus prepared may contain coarse particles which may be retained on a screen of 48 mesh having a mesh opening of 0.297 mm according to JIS Z 8801 in the test of the particle size distribution. Such foamy resin particles are sometimes responsible for the scale deposition on the walls of the polymerization reactor in the upper part to seriously decrease the productivity.
Various means, of course, are undertaken to reduce the volume of foams on the polymerization mixture. For example, the foams can be destroyed mechanically by rotating foam-breaking blades provided on the stirrer shaft. This method is not always very effective in practice because of the deposition of the foam debris on to the foam-breaking blades and inner walls of the polymerization reactor resulting in scale deposition.